Process for the preparation of isothiocyanates

Abstract

There is disclosed a process for the preparation of isothiocyanates of formula ##STR1## wherein R 1 and R 2 are each independently of the other C 1 -C 6 alkyl and R 3 , R 4 and R 5 are each independently of the other hydrogen, halogen, C 1 -C 4 alkyl, C 1 -C 4 alkoxy, trifluoromethyl or nitro, which process comprises an amine of formula ##STR2## with ammonium thiocyanate or an alkali metal thiocyanate, in the presence of acid and an inert solvent, to the thiourea of formula ##STR3## and cleaving said thiourea by heating. The essential feature of this process consists in carrying out the reaction of the amine of the above formula with ammonium thiocyanate or an alkali metal thiocyanate in the presence of 0.5-5% by weight of water, based on the total weight of the reaction mixture. The isothiocyanates of the above formula are intermediates for the synthesis of pesticidal compounds.

What is claimed is: 
     
       1. A process for the preparation of an isothiocyante of formula I ##STR10## wherein R 1  and R 2  are each independently of the other C 1  -C 6  alkyl and R 3 , R 4  and R 5  are each independently of the other hydrogen, halogen, C 1  -C 4  alkyl, C 1  -C 4  alkoxy, trifluoromethyl or nitro, which process comprises reacting an amine of formula II ##STR11## wherein R 1 , R 2 , R 3 , R 4  and R 5  are as defined for formula I, in the presence of at least the equivalent amount of an acid and in the presence of 0.5-5% by water, based on the total weight of the reaction mixture, in an inert solvent, with ammonium thiocyanate or an alkali metal thiocyanate to a thiourea of formula III ##STR12## wherein R 1 , R 2 , R 3 , R 4  and R 5  are as defined for formula I, and cleaving said thiourea by heating to give an isothiocyanate and ammonia. 
     
     
       2. A process according to claim 1, wherein R 1  and R 2  are each independently of the other C 2  -C 4  alkyl, R 3  is hydrogen, chloro, methoxy, trifluoromethyl or nitro, and R 4  and R 5  are each hydrogen or chloro. 
     
     
       3. A process according to claim 1, wherein R 1  and R 2  are each isopropyl or sec-butyl, R 3  is hydrogen, chloro or trifluoromethyl, R 4  is hydrogen or chloro, and R 5  is hydrogen. 
     
     
       4. A process according to claim 1, wherein R 1  and R 2  are isopropyl and R 3 , R 4  and R 5  are hydrogen. 
     
     
       5. A process according to claim 1, wherein the reaction of the amine of formula II is carried out with ammonium thiocyanate or an alkali metal thiocyanate, in the presence of 1-1.25 equivalents of a strong non-oxidising mineral acid per mol of amine of formula II. 
     
     
       6. A process according to claim 5, wherein 1-1.25 equivalents of hydrochloric acid, hydrobromic acid or sulfuric acid are used per mol of amine of formula II. 
     
     
       7. A process according to claim 5, wherein 1.05-1.15 mol of hydrochloric acid is used per mol of amine of formula II. 
     
     
       8. A process according to claim 1, wherein the amine is used in the form of the hydrochloride. 
     
     
       9. A process according to claim 1, wherein the reaction of an amine of formula II with ammonium thiocyanate or an alkali metal thiocyanate is carried out in an aromatic hydrocarbon or an aliphatic or aromatic halogenated hydrocarbon as solvent. 
     
     
       10. A process according to claim 9, wherein the reaction of an amine of formula II is carried out with ammonium thiocyanate or an alkali metal thiocyanate in toluene, xylene or a mixture of alkylbenzenes as solvent. 
     
     
       11. A process according to claim 9, wherein the reaction of an amine of formula II with ammonium thiocyanate or an alkali metal thiocyanate is carried out in xylene or a mixture of alkylbenzenes having a boiling range from 150° to 170° C. 
     
     
       12. A process according to claim 1, wherein the reaction of the amine of formula II with the ammonium thiocyanate or alkali metal thiocyanate is carried out in the presence of 1-2% by weight of water, based on the total weight of the reaction mixture. 
     
     
       13. A process according to claim 1, wherein the amine of formula II is reacted with sodium thiocyanate. 
     
     
       14. A process according to claim 1, wherein 1.05-1.25 mol of ammonium thiocyanate or alkali metal thiocyanate is used per mol of amine of formula II. 
     
     
       15. A process according to claim 1, wherein the amine of formula II is reacted with the ammonium thiocyanate or alkali metal thiocyanate in the temperature range from 80° to 120° C. 
     
     
       16. A process according to claim 15, wherein the reaction of the amine of formula II with the ammonium thiocyanate or alkali metal thiocyanate is carried out in the temperature range from 90° to 110° C. 
     
     
       17. A process according to claim 1, wherein the thermal cleavage of the thiourea of formula III is carried out in the temperature range from 125° to 170° C. 
     
     
       18. A process according to claim 17, wherein the cleavage of the thiourea of the formula III is carried out in the temperature range from 140° to 160° C. 
     
     
       19. A process according to claim 1, wherein the cleavage of the thiourea of formula III is carried out under a pressure of 0.8-1 bar. 
     
     
       20. A process for the preparation of a compound of formula Ia ##STR13## wherein R' 1  and R' 2  are each isopropyl or sec-butyl, R' 3  is hydrogen, chloro or trifluoromethyl and R' 4  is hydrogen or chloro, which comprises reacting an amine of formula IIa ##STR14## wherein R' 1 , R' 2 , R' 3  and R' 4  are as defined for formula Ia, in the temperature range from 80° to 90° C., in toluene, xylene or a mixture of alkylbenzenes as solvent, and in the presence of 1.05 to 1.15 mol of hydrogen chloride per mol of amine of formula IIa, with 1.05 to 1-25 mol of sodium thiocyanate per mol of amine of formula IIa, to the thiourea of formula IIIa ##STR15## wherein R' 1 , R' 2 , R' 3  and R' 4  are as defined for formula Ia, and cleaving said thiourea of formula IIIa by heating in the temperature range from 140°-160° C., in xylene or a mixture of alkylbenzenes, to the isothiocyanate of formula Ia.